|De Souza, Jorge|
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2007
Publication Date: 9/15/2007
Citation: Roberts, D.P., Mckenna, L.F., Lakshman, D.K., Meyer, S.L., Kong, H.N., De Souza, J.T., Lydon, J., Baker, C.J., Buyer, J.S., Chung, S. 2007. Suppression of damping-off of cucumber caused by pythium ultimum with live cells and extracts of serratia marcescens n4-5. Soil Biology and Biochemistry. 39:2275-2288. Interpretive Summary: Soilborne plant pathogens cause diseases that result in major economic losses to farmers in the United States. Biological control measures for these diseases need to be developed due to environmental problems associated with existing chemical controls. We demonstrated that the bacterium Serratia marcescens strain N4-5 is effective in suppressing damping-off of cucumber and other cucurbits caused by the important soilborne pathogen Pythium ultimum. We also demonstrated that cell-free extracts from cell mass of strain N4-5 effectively suppressed damping-off of cucumber caused by P. ultimum when applied as a seed treatment. Biochemical characterization of these extracts strongly suggested that the antibiotic prodigiosin is important for this suppression. It is likely that obtaining U.S. EPA registration for the use of certain live beneficial bacteria, such as S. marcescens, will prove difficult due to the perceived risks to human health that associated with them. However, seed treatments with cell-free extracts of S. marcescens should allow the use of this bacterium for suppression of P. ultimum without the perception of risks to human health. This information will be useful to scientists devising strategies for biological control of soilborne plant pathogens.
Technical Abstract: Environmentally friendly control measures are needed for the soilborne pathogens Pythium ultimum and Meloidogyne incognita. These pathogens can cause severe losses to field- and greenhouse-grown cucumber and other cucurbits. Live cells and ethanol extracts of cultures of the bacterium Serratia marcescens N4-5 provided significant suppression of damping-off of cucumber caused by P. ultimum when applied as a seed treatment. Live cells of this bacterium also suppressed damping-off caused by P. ultimum on cantaloupe, muskmelon, and pumpkin. Culture filtrates from strain N4-5 contained chitinase and protease activities while ethanol extracts contained the antibiotic prodigiosin, the surfactant serrawettin W1, and possibly other unidentified surfactants. Production of prodigiosin and serrawettin W1 was temperature-dependent, both compounds being detected in extracts from N4-5 grown at 28oC but not in extracts from N4-5 grown at 37oC. Ethanol extracts from strain N4-5 grown at 28oC inhibited germination of sporangia and mycelial growth by P. ultimum in in vitro experiments. There was no in vitro inhibition of P. ultimum associated with ethanol extracts of strain N4-5 grown at 37oC. Prodigiosin, purified from two consecutive thin layer chromatography runs using different solvent systems, inhibited germination of sporangia and mycelial growth of P. ultimum. Another unidentified compound(s) also inhibited germination of sporangia but did not inhibit mycelial growth. There was no in vitro inhibition associated with serrawettin W1. Culture filtrates from strain N4-5 suppressed egg hatch of M. incognita in in vitro assays. However, live cells of strain N4-5 applied as a seed treatment and drench failed to suppress populations of this nematode in cucumber rhizosphere in greenhouse pot experiments. These results demonstrate that live cells and cell-free extracts of S. marcescens N4-5 are effective for suppression of damping-off cucumber caused by P. ultimum possibly due in part to the production of the antibiotic prodigiosin.